Method for mass-spectroscopic examination of a gas mixture and mass spectrometer intended for carrying out this method

Abstract

For mass-spectroscopic examinations of gas mixtures a mass spectrometer is used which comprises a quistor in which ions of the gas mixture whose charge-to-mass ratio is located in a predetermined range are stored by generating an electromagnetic field. By varying the field parameters, the ions are forced successively to leave the ion trap. The intensity of the ion flow leaving the ion trap is measured as a function of the variation of the field parameters. For improving the resolution, one uses a quistor of the type where the distance-related ratio Q of the radii of the inscribed vertex circles of the electrodes comply with the condition Q≦3.990, wherein ##EQU1## R e being the radius of the cross-section of the vertex of the end electrodes (3,5); R r being the radius of the cross-section of the vertex of the annular electrode (4); z o being the distance between the vertex of each end electrode (3,5) and the center of the quistor; and r o being the distance between the vertex of the annular electrode (4) and the center of the quistor.

We claim: 
     
       1. A method for mass-spectroscopic examination of a gas mixture using a mass spectrometer comprising an ion trap designed as quistor with an annular electrode defining a chamber and two end electrodes closing the chamber defined by the annular electrode, at least one of the said end electrodes being provided with a performation forming the extension of the axis of rotation of the annular electrode, the method comprising the steps of: applying to the annular electrode an rf voltage of am amplitude and frequency and, if necessary, a direct potential convenient to generate within the ion trap a three-dimensional rf quadrupole field suited to catch and store in the ion trap ions having a charge-to-mass ratio situated within a predetermined range;   introducing or generating ions of the gas mixture into, or in the ion trap and storing therein those ions whose charge-to-mass ratio is situated within the predetermined range;   varying at least one of the field parameters consisting of the amplitude, the frequency and, if applicable, the direct potential, in such a manner that ions whose charge-to-mass ratio varies monotonously become successively instable and leave the said ion trap in the direction of the axis of rotation of its annular electrode and through the said perforation in the said end electrode; and   measuring and recording the intensity of the ion flow leaving the ion trap as a function of the variation of the field parameters, characterized in that   for carrying out the method a quistor is used in which the distance-related ration Q of the radii of the inscribed vertex circles of the electrodes comply with the condition Q≦3.990, wherein ##EQU4## R e  being the radius of the cross-section of the vertex of the said end electrodes;   R r  being the radius of the cross-section of the vertex of the said annular electrode;   z o  being the distance between the vertex of each said end electrode and the center of the said quistor; and   r o  being the distance between the vertex of the said annular electrode and the center of the said quistor.   
     
     
       2. A mass spectrometer comprising an ion trap designed as quistor with an annular electrode defining a chamber and two end electrodes closing the chamber defined by the annular electrode, at least one of the end electrodes being provided with a perforation forming the extension of the axis of rotation of the annular electrode, suited for examination of a gas mixture, characterized in that the distance-related ratio Q of the radii of the inscribed vertex circles of the electrodes comply with the condition Q≦3.990, wherein ##EQU5## R e  being the radius of the cross-section of the vertex of the said end electrodes;   R r  being the radius of the cross-section of the vertex of the said annular electrode;   z o  being the distance between the vertex of each said end electrode and the center of the said quistor; and   r o  being the distance between the vertex of the said annular electrode and the center of the said quistor.   
     
     
       3. A mass spectrometer according to claim 2, characterized in that of the dimensions of the quistor which determine the distance-related ratio Q, the distance r o  of the vertex of the annular electrode from the center of the said quistor is equal to a value which guarantees that the greatest interesting mass is still trapped by the storage field, at the amplitude of the rf voltage applied to the annular electrode, the distance z o  of the vertex points of the end electrodes from the center of the quistor is equal to Q z o  =r o  /4√Q, for given ratio Q, and the radii R e  and R r  of the vertex cross-sections are selected in such a manner that R e  ×R r  =r o  ×z o .